1. Field of the Invention
The invention relates to micromachined acoustic device packages and in particular to micromachined miniature MEMS microphone packages and fabrication methods thereof.
2. Description of the Related Art
Micromachined silicon microphones have been disclosed in many patents. For example, U.S. Pat. Nos. 5,619,476, 5,870,351, 5,894,452 and 6,493,288 disclose capacitive-type ultrasonic transducer fabrication methods. U.S. Pat. Nos. 5,146,435; 5,452,268; 6,535,460 and 6,870,937 disclose micromachined capacitive transducers that are mainly used for sound pickups. In these patents, however, inventiveness has focused on the design and manufacturing of micromachined microphone dies. In other words, inventiveness has focused on the wafer level processing of microphones.
For microphones to be used in any type of electronic devices, proper housing needs to be provided such that the microphone dies can be housed in a suitable package to prevent it from environmental interferences. Preferably, this housing structure also shields the sensing elements of a silicon microphone from outside electromagnetic interferences. Also, the packaged microphones need to have contact leads, such that the contact leads can be soldered onto an electronic board where they are used. Lastly, the packaging method used for the microphones, must be low cost and allow for mass production.
Compared with traditional electret microphones, micromachined MEMS microphones have the advantage of being able to sustain high re-flow temperatures. Thus, to minimize electronic products assembly costs, the micromachined MEMS microphones are packaged in form factors that allows for the surface mounting of microphones to a PCB board.
Several packaging methods for MEMS microphones have been disclosed. U.S. Pat. No. 6,781,231, the entirety of which is hereby incorporated by reference, discloses a micro-electro-mechanical system package including a micro-electro-mechanical system microphone, a substrate, and a cover. The substrate has a surface for supporting the micro-electro-mechanical microphone. The cover includes a conductive layer having a center portion bounded by a peripheral edge portion. A housing element is formed by connecting the peripheral edge portion of the cover to the substrate. The center portion of the cover is spaced from the surface of the substrate to accommodate the micro-electro-mechanical system microphone. The housing includes an acoustic port for allowing an acoustic signal to reach the micro-electro-mechanical system microphone. U.S. Patent application publication 2005/0018864, the entirety of which is hereby incorporated by reference, discloses a silicon condenser microphone package comprising a transducer unit, a substrate, and a cover. The substrate includes an upper surface having a recess formed therein. The transducer unit is attached to the upper surface of the substrate and overlaps with at least a portion of the recess wherein a back volume of the transducer unit is formed between the transducer unit and the substrate. The cover is placed over the transducer unit and includes an aperture.
U.S. Pat. No. 7,434,305, the entirety of which is hereby incorporated by reference, discloses a silicon condenser microphone package comprising a transducer unit, substrate, and a cover. The substrate includes an upper surface having a recess formed therein. The transducer unit is attached to the upper surface of the substrate and overlaps with at least a portion of the recess wherein a back volume of the transducer unit is formed between the transducer unit and the substrate. The cover is placed over the transducer unit and includes an aperture.
U.S. Pat. No. 7,439,616, the entirety of which is hereby incorporated by reference, discloses a silicon condenser microphone package including a transducer unit, a substrate, and a cover. The substrate, including an upper surface transducer unit, is attached to the upper surface of the substrate and overlaps with at least a portion of the recess wherein a back volume of the transducer unit is formed between the transducer unit and the substrate. The cover is placed over the transducer unit and either the cover or the substrate includes an aperture.
The mentioned packaging methods provide a silicon condenser microphone package that allows acoustic energy to contact a transducer disposed within a housing. The housing provides necessary pressure references, while at the same time, protects the transducer from light, electromagnetic interference and physical damage. The mentioned packaging methods, however, fail to critically address aspects of packaged microphones related to usage and assembly thereof. Some aspects include, but are not limited to, acoustic leakage through sidewalls and/or a cover of the microphone package, secure microphone attachment to an underlying PCB board, effectiveness of shielding from electromagnetic interference, electronic signal transmission distortion from the microphone package to the underlying PCB board, flexibility of a packaged microphone for surface mounting; and ease of manufacturing for volume production, etc.